Method for heat treating a polar, dielectric parison

ABSTRACT

A method of heat treating an axial zone of a polar, dielectrically lossy, thermoplastic, polymeric parison with a high frequency energy source to produce a temperature gradient across the thickness of the sidewall in said zone prior to blow molding the parison into a bottle, whereby the temperature is higher on the inner surface of the parison sidewall than on the outer surface of the parison sidewall.

United States Patent [191 Silver-man METHOD FOR HEAT TREATING A POLAR,

DIELECTRIC PARISON [75] Inventor: Alan Silverman, Bridgewater Township,NJ.

[73] Assignee: American Can Company,

Greenwich, Conn.

22 Filed: Mar. 29, 1973 21 Appl. No.: 346,172

[ Jan. 15, 1974 3,594,862 7/1971 Seefluth 425/1744 PrimaryExaminer-Bruce A. Reynolds Attorney-Robert P. Auber et a1.

[57 ABSTRACT A method of heat treating an axial zone of a polar, dielectrically lossy, thermoplastic, polymeric parison with a highfrequency energy source to produce a temperature gradient across. thethickness of the sidewall in said zone prior to blow molding the parisoninto a bottle, whereby the temperature is higher on the inner surface ofthe parison sidewall than on the outer sur- [56] References Cited faceof the parison sidewall.

UNITED STATES PATENTS 3,462,582 8/1969 Cines 219/10.43 18 Claims, 3Drawing Figures 1;, N g *i e 15 e s a Q a x Q g R It k 1 i s k x. n g u,Q Q Q E T k U k l my R1 '5 o E, m 1 Q o 2/ 1, Q k, k Q lq Q W Y s '3 ifR s 3 &% e: QK E We Na. sf RI 1 l (f 0. II/ 75)' 0. 798- fl. 992- I./3'Z-//fl F pi WI, VIII/A V l-ml 495- a 52; 0. w/. m

252--.2i2 Lwm0 MM 4/1964 Gewecke 425/l74.8

PATENTEDJAH 151974 mm 1 HF 2- i Li SHEET 2 BF 2 PATENTEUJAN & 5 I974 r rr 1 METHOD FOR I'IEAT TREATING A POLAR, DIELECTRIC PARISON BACKGROUND OFTHE INVENTION The subject invention relates to heat treating parisons tobe blow molded into molecularly oriented bottles and more particularlyto a heat treatment of polar, dielectrically lossy parisons prior tosaid blow molding wherein a temperature gradient is imparted to an axialzone in the sidewall of the parison.

Molecular orientation of thermoplastic polymeric materials is not new.Molecularly oriented film and sheet are widely used and have improvedphysical properties, including superior impact resistance,'increasedresistance r to creep, increased stiffness, increased resistance tostress rupture and reduced stress crazing, when compared to theirunoriented counterparts. Examples of such materials are given in US Pat.No. 3,141,912.

For a given polymer and end use application, there is an optimum levelof orientation as determined by orientation releasestress (OR S), whichmay be below the maximum possible orientation level. For example, impactstrength may reach a maximum value as the amount of orientation isincreased, with additional orientation resulting in a decreased impactstrength. Another example of a property which may deteriorate withattempts to achieve high levels of orientation is optical transparency;certain polymers stress whiten, giving them a milky appearance.

The amount of orientation in an article formed from a polymeric materialis affected by the conditions under which the material is oriented; Forexample, in

a tubular article higher levels of circumferential orientation resultfrom increasing the amount of stretch in either the circumferential oraxial direction, by increasing the stretching rate, and by decreasingthe stretching temperature.

It is known to form plastic bottles by blow-molding a parison, ofclosed-end tube. While such techniques have met with some success,generally it has not been economically practicable to form bottles forcarbonated beverages bythis technique. The reason has been.

that if the bottle is oriented, by stretching, sufficiently to developthe properties required of containers for carbonated beverages (assuminga wall thickness thin enough to be economic), stress whitening has beenobserved to occur, making the container unsalable. Impact strength isalso found to be undesirably low.

Further analysis of this phenomenon has brought the realization thatstress whitening, which develops primarily at the inner surface portionof the bottle wall, is due to the fact that the inside of the parison isstretched to a much higher extent, proportionally, than the outside. Ithas been found that the degree of orientation is not constant across thebottle wall thickness, but on the contrary varies substantially acrossthe wall, and at or near the inner surface portion of the wall issufficiently high to give rise to thestress whitening.

Accordingly, a method of heat treatment is disclosed in co-pendingapplication Ser. No. 319,380, filed Dec. 29, 1972, for achieving a moreuniform circumferential orientation across the thickness of the bottlesidewall by imparting a radial temperature gradient to an axial zone ofthe sidewall of the parison prior to the parison being blow molded intoa bottle whereby, in said zone, the inner surface of the parison is madehotter than the outer surface of the parison. The entirety of saidcopending application is hereby incorporated by reference.

In order for such a heat 'treating'process to be economical, it must berapid. The instant invention meets the economic as well as thetechnological requirements associated with producing a temperaturegradient in an axial zone of a polar, dielectrically lossy parisonsidewall prior to blowing of the parison into a bottle.

SUMMARY OF THE INVENTION BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is acentral, vertical sectional view ofconcentrically arranged electrodesand the parison to be heated therebetween.

FIG. 2 is a sectional view on the horizontal plane inidcated by the line2-2 of FIG. 1.

FIG. 3 is a partial central vertical sectional view and partialelevational view of a bottle blow-molded from the parison shown in FIG.1.

DESCRIPTION OF THE PREFERRED EMBODIMENT of stretch of the inner andouter surfaces of the parison Y in the blowing operation, by thestretching rate, and by the average temperature of thermoplasticmaterial during the blowing operation. The temperature gradient isgreater for a large relative amount of stretch, a lower stretching rateand a higher blowing temperature.

The amount of stretch of the inner surface of the parison during theprocess of forming the parison into a blow-molded article may beexpressed in terms of an inside stretch ratio," SR(i), which is theratio of the inside diameter of the blown article at any given axiallocation to the inside diameter of that portion of the parison which wasformed into the article at the axial location where the articlesdiameter was measured. The amount of stretch of the outer surface of theparison may similarly be expressed in terms of an outside stretch ratio,SR(o). In the case of a non-circular cross-sectional shape of theparison or the blownarticle, the effective diameter may be used toobtain the respective stretch ratios.

It can easily be shown that the inner surface of the parison sidewallis, in fact, stretched more than the outer surface of the parisonsidewall in the course of blowing the bottle. The extent to which SR(i)is greater than SR(o) is a measure of the relative amounts of stretch ofthe inner and outer surfaces of the parison.

If, during the blowing operation, there is a uniform temperature acrossthe thickness of the sidewall of the parison, the inside portion of theblown articles sidewall will be substantially more highly oriented thanthe outside because, relative to the outside portion, the inside isstretched to a greater extent.

The parison is heat treated in accordance with the present invention tocompensate for the relative difference in amounts of stretch of theparison sidewall from the inner to the outer portions thereof. Sinceless orientation occurs, for a given amount of stretch, at a higherstretching temperature, a temperature gradient is imparted to theparison sidewall prior to the blowing operation, with the temperature ofthe inner surface being greater than that of the outer surface.

It has been found that, in order to produce a blown bottle in which themaximum circumferential ORS at any axial location in the bottle sidewallis less than about twice the minimum ORS at that location, thetemperature gradient across the parison sidewall, in Farenheit degrees,at the corresponding axial location in the parison, should be fromabout:

A preferred range of the temperature gradient in Farenheit degress isfrom about:

to about:

For the parison of the type illustrated in FIG. 1, the most preferredtemperature gradient, in Farenheit degrees, in an axial zone of theparison is determined according to the formula:

For purposes of this specification, the orientation release stress isdetermined according to an adaptation of ASTM Test D 1504. In thismethod, bottles are first conditioned at 72 F. (i 5 F.) at 50 percentrelative humidity (i percent) for about 6 hours. The bottle specimensare prepared as follows:

The tops and bottoms of a given bottle are removed by cutting with aband saw. Annular rings of approximately /a inch width and approximatelyto 30 mils thick in an axial zone are cut off with a lathe in sequencefrom the resulting cylindrical section of the bottle wall. After theedges of each annulus are filed to remove flash material, the maximumand minimum thicknesses of each are measured in the region to beanalyzed.

To obtain inside specimens which will provide information of the averagecircumferential direction orientation near the inner surface of thebottle wall, an annulus is slipped over a mandrel mounted on a lathe andmaterial is removed from the outside surface in 2.5 mil steps, therebyresulting in an annulus thickness of about 10 mils. The lathe isoperated at a lineal speed of 250 feet per minute at the cutting tool.The last few mils of material are always removed on a milling machineaccording to the procedure described below.

To obtain outside specimens from which the average circumferentialdirection orientation near the outer surface of the bottle wall can bedetermined, an annulus is slipped into a collet mounted on the lathe andthe material is removed from the inside surface in 2.5 mil steps, togive an annulus thickness of about 10 mils. An additional few mils ofmaterial are then removed on a milling machine.

To obtain middle specimens which provide a measure of the averagecircumferential direction orientation midway through the thickness ofthe bottle sidewall, material is first removed from the inside of theannulus as set forth above to give a thickness of 15 to 20 mils. Analmost equal amount of material is then removed from the outside of theannulus as set forth above to give middle specimens approximately 10mils thick.

The final step in sample preparation is milling of the annuli to assurereasonably uniform cross-sections. This is accomplished by cutting eachof these annuli so that the resulting three strips can be mounted viadouble-faced masking tape onto an aluminum block previously locked ontothe table of the milling machine and faced off to assure parallelpositioning of the samples to be milled. The milling operation isperformed on the specimens by removing only about 1 mi] of material perpass unitl 1 mil from the required 6 to 7 mil thickness, followed by 1/3mil steps so that the desired thickness is achieved. The slowest machinecross head speed, 9/16 inches/minute, is used in conjunction with thetwo-fluted end mill 3/4 inch in diameter, rotating at l rpm. The threespecimens are then stripped from the mounting plate, cut into minimum 1inch lengths and the maximum and minimum thickness measured with amicrometer. These specimens are now ready for the actual measurement ofORS according to ASTM Test D 1504. In the modified procedure employedherein, samples are immersed in a 133 silicone oil bath.

Parisons heat treated by the method of this invention can be producedfrom any polar or dielectrically lossy thermoplastic, such as polyvinylchloride, certain polyesters, and the so-called barrier resins, and moreparticularly from those which are glassy and noncrystalline at roomtemperature, such as the previously mentioned polyvinyl chloride and theso-called barrier resins. Dielectrically lossy thermoplastics are, forthe purpose of this specification, to be defined as those whosedielectric loss tangents are greater than 0.02 at room temperaturemeasured at 30 megacycles per second.

The present invention is particularly applicable to the production ofplastic bottles containing fluids under a high internal pressure, suchas, for example, beer, carbonated beverages and aerosol containerproducts. Such bottles require that the polymeric material from whichthe bottle is formed have a low permeability to gases such as carbondioxide.

Polymers suitable for blowing into bottles are prepared by polymerizinga major portion of an olefinically unsaturated nitrile, such asacrylonitrile, and a .minor portion of an ester of an olefinicallysaturated carboxylic acid, such as ethyl acrylate, in the presence of arubber containing a major proportion of a conjugated diene monomer, suchas butadiene, and a minor proportion of an olefinically unsaturatednitrile, such as acrylonitrile.

The conjugated diene monomers useful in the preparation of such polymersinclude 1,3-butadiene, isoprene, chloroprene, bromoprene, cyanoprene,2,3dimethyl-l ,3-butadiene, 2-ethyl- 1 ,3-butadiene,2,3-diethyl-l,3-butadiene and the like.

The olefinically unsaturated nitriles useful in the preparation of suchpolymers are the alpha,betaolefinically unsaturated mononitriles havingthe structure wherein R is hydrogen, a lower alkyl group having from 1to 4 carbon atoms, or a halogen. Such compounds include acrylonitrile,alpha-chloroacrylonitrile, alphafluoroacrylonitrile, methacrylonitrile,ethacrylonitrile, and the like.

The esters of olefinically unsaturated carboxylic acid useful in thepreparation of such polymers are preferably the lower alkyl esters ofalpha,beta'olefinically unsaturated carboxylic acids and more preferredare the esters having the structure wherein R is hydrogen, an alkylgroup having from 1 to 4 carbon atoms, or a halogen, and R is an alkylgroup having from 1 to 6 carbon atoms. Compounds of this ty e ifieludefiiethyl acrylate, etnyracryisizjtne propyl acrylates, the butylacrylates, the pentyl acryl ates, and the hexyl acrylates, methylmethacrylate, ethyl methacrylate, the propyl methacrylates, the butylmethacrylates, the pentyl methacrylates, and the hexyl methacrylates,methyl alphachloroacrylate, ethyl al pha-chloroacrylate and the like.

The more preferred polymers are derived from (A) about 60 to 90 parts byweight of an alpha,betaolefinically unsaturated mononitrile having thestructure CH C(--R,)--CN where R, is selected from the group consistingof hydrogen, halogen, and the lower alkyl groups, (B) about 40 to partsby weight of an ester of an olefinically unsaturated carboxylic acidhaving the structure CH IQT R Q9119 1 2. Wh 3. .1. 2 fiPP above and -Ris an alkyl group having from I to 6 carbon atoms, (A) and (B) togethercomprising 100 parts by weight, polymerized in the presence of (C) about1 to parts by weight of a nitrile rubber containing about 60 to 80 percent by weight of moieties derived from a conjugated diene monomer andabout to 20 percent by weight of moieties derived from a mononitrilehaving said CH C(-R,)CN structure.

The most preferred polymels are derived from about to 90 parts by weightof acrylonitrile or methacyrlonitrile and about 40 to 10 parts by weightof an ester selected from the group consisting of methyl acrylate, ethylacrylate and methyl methacrylate, polymerized in the presence of about 1to 20 additional parts by weight of a nitrile rubber containing about 60to 80 percent by weight butadiene or isoprene moieties and about 40 to20 percent by weight of acrylonitrile or methacrylonitrile moieties.

More specifically, the most preferred polymers are derived from about 73to 77 parts by weight acrylonitrile and 27 to 23 parts by weight methylacrylate, polymerized in the presence of 8 to 10 additional parts byweight of a nitrile rubber containing about percent by weight butadienemoieties and about 30 percent by weight acrylonitrile moieties.

Further examples of such polymers may be found in U.S. Pat. No.3,426,102, the entirety of which is hereby incorporated into the instantspecification by reference.

The instant invention is best utilized in the heat treating of a parisonto be blow molded into a bottle suitable for containing beer orcarbonated beverage. Accordingly, the parison 11 shown in FIG. 1 to beblown into a bottle 12 shown in FIG. 3 is formed by injection molding athermoplastic polymer derived from parts by weight acrylonitrile and 25parts by weight methyl acrylate polymerized in the presence of 9additional parts by weight of a nitrile rubber containing about 70percent by weight 1,3-butadiene and about 30 percent by weightacrylonitrile. Injection molding is a technique used for forming aparison having a particular material distribution, as shown in FIG. 1,but any of the conventional parison forming techniques'may be employed.

The next step comprises a novel heat treatment wherein a temperaturegradient is imparted across the thickness of the sidewall 13 of theparison 1 1 by means of high frequency electromagnetic energy wherebythe parison sidewall 13 becomes hotter on its inner surface 17 than onits outer surface 19. Preferably, the frequency is radio frequency, andthe most preferred frequency range is between 10 and 10 cycles persecond. An inner electrode 21, which is also the blowing core pin,having apertures 27 for high pressure fluid utilized in the subsequentblow molding, is located within the parison 11 (which is a polar, lossydielectric), and is connected, by means of a source 23 of radiofrequency energy to an outer electrode shell 25 which surrounds theparison 11. The dissipation of radio frequency energy by the dielectriccauses localized heating of the parison 11, which increases along aradial path inwardly toward the inner parison surface 17.

More particularly, for parisons of circular crosssection, the rate ofdissipation of energy per unit volume of polymer at any point isinversely proportional to the distance measured radially outward to thatpoint from the long axis of the parison 11. This rate is alsoproportional to the voltage applied to the electrodes 21 and 25 and tothe specific dielectric properties of the polymeric parison and of thedielectric or dielectrics which occupy the spaces between the parison 11and the electrodes 21 and 25.

It should also be noted that the axial temperature profile of theparison will generally be so varied as to accomodate the axially varyingthickness of the parison sidewall and to control the manner in which theparison inflates. The method of this invention is particularlywell-suited to imparting such axial temperature variations to theparison, inasmuch as the relative spacing between the parison 11 and theconcentric electrodes 21 and 25 along the parison axis can be adjustedto provide the desired temperatures. The radial temperature gradientsalong the long axis of the parison will vary accordingly. It can beshown that, to a first approximation, the value of the changing radialgradient along the axis of the parison is proportional to thecorresponding change in the relative differences in stretching betweenthe inside and outside of the parison sidewall.

The-heat treating process described above is dependent upon the properchoice of electrode dimensions. For a parison of given dimensions anddielectric properties, suitable electrode dimensions may be determinedby experimental trial and error. Such an approach, however, isexceedingly time consuming and laborious. We have found a more efficientapproach to the problem of determining electrode dimensions by utilizinga mathematical equation which describes the amounts of power dissipatedin the dielectric parison. This equation, when coupled with a suitableheat transfer analysis, approximately predicts the degree of heatingwithin the dielectric parison for any electrode configuration subject tothe qualifications noted below. In particular, most parisons geometriesapproximate right circular cylindrical annuli which may be closed at oneend. The equation assumes, therefore, that the dielectric parison is aright circular cylindrical annulus interposed between electrodes whichare concentric right circular cylinders. Owing to imperfect knowledge ofthe physical and electrical properties of the dielectric material and tothe aforementioned approximations, electrode dimensions predicted bythis equation must be corrected experimentally.

The power dissipation per unit volume, at any radial point withindielectric materials in annular arrangement between concentric, rightcircular cylindrical electrodes is given by the equation:

h In (ml/m] O H J J P Power dissipation/unit volume in the n' annulardielectric layer V applied voltage (root mean square) w angularfrequency of applied electric field s permittivity of free space 6,,dielectric constant of the n'" annular dielectric layer (tan 8,,) losstangent of the n'" annular dielectric layer tan 6,, wC' (tan 5 +1) andfor which C 21re,,e,,L/l,, (r /r capacitance of n' annular dielectriclayer where L= a measure of length along the common axis of the rightcircular cylindrical electrodes. Also:

ReZ, ReZ, ReZ ReZ,,+ .+ReZ,,

And:

ImZ, ImZ ImZ +ImZ ImZ Where N total number of annular layers.

The parison 11 with the temperature gradient produced by the method ofthis invention is then blowmolded in the leathery state with anincreasing pressure to a maximum pressure which reaches about 200 p.s.i.in about 15 seconds to produce the bottle 12 whose sidewall 14 has anaverage circumferential orientation release stress which issubstantially uniform radially when compared to bottles blown fromsimilar parisons heated conventionally.

The above procedure may be utilized to produce various average levels oforientation. Average circumferential orientation levels between 350 and2500 p.s.i., and especially between 500 and 1600 p.s.i. achievedutilizing the temperature gradient, may be realized which aresubstantially uniform radially across the sidewall of the bottle.

That a temperature gradient can be produced in a polar, dielectricthermoplastic polymer by the method described above is demonstratedbelow. A parison of the type shown in FIG. 1 was injection molded from apolymer comprised of 75 parts by weight acrylonitrile and 25 parts byweight methyl acrylate polymerized in the presence of 9 additional partsby weight of a nitrile rubber containing about percent by weight 1,3-butadiene and about 30 percent by weight acrylonitrile.

The heating apparatus employed was arranged approximately as shown inFIG. 1, the electrodes being made of half-hard brass. The source 23 ofhigh frequency energy was a model L14E generator supplied by the NewJersey Electronic Corp., Clifton, New Jersey. The parison was heattreated at about 42 percent of the maximum power setting at 30megacycles for about 12 seconds. This power setting is believed togenerate a voltage applied across the electrodes 21 and 25 on the orderof 1500 2000 volts.

The parison was provided with three holes so located as to permitmeasurement by thermocouples of the inside, middle and outside sidewalltemperatures of the parison at a point about 5 inches from the open endof the parison shown in FIG. 1. The holes, which were of diameter 0.030inch, were located on circles midway between and 0.025 inch from theinner and outer sidewall surfaces. Since total thickness of the parisonemployed was 0.190 inch at the place these holes were located, thethickness over which the temperature difference was measured wastherefore 0.140 inch. The parison was heat treated as described aboveand thermocouples as soon thereafter as possible were inserted into theholes. The earliest data thus measured correspond to approximately 12 15seconds after completion of the heat treatment step, and are given inTable I below.

More important are the temperatures in the earlier seconds after heattreatment during which blowmolding is initiated, which temperature arenot amenable to direct measurement. Calculations from heat transferequations, however, provide the necessary information, and these datalikewise are presented in the table below. It can be seen that aninitial radial temperature gradient of over 80 F. was obtained acrossthe thickness of0.l40 inches, or about 06 F. per mil.

TABLE I TEMPERATURE GRADIENTS IN A PARISON HEATED BY RADIO FREQUENCYENERGY USING CONCENTRIC ELECTRODES v Sec. 12 Sec. Sec.

After After After Initial Heating Heating Heating Calculated Inside 75237 206 Temp. Middle 75 188 183 (F) Outside 75 155 166 Measured Inside75 206 198 Temp. Middle 75 I88 189 (F) Outside 75 l70 Employing thegiven equation concurrently with a heat transfer analysis, a set ofelectrodes 21 and 25 as seen in FIG. 1 was designed for the parison llof FIG. 1 wherein temperatures within the parison 11 were establishedsuitable for subsequent inflation to form a bottle whose sidewall ismore uniformly oriented than would be obtained from blow-molding asimilar bottle from a similar parison conventionally heated. The innerdiameter of the outer electrode 25 was assumed to be everywhere largerby 0.240 inches than the outer diameter of the parison 11. Then, forestimated dimensions for the central electrode, as shown in Table II,for a voltage of 3,300 volts'applied across the electrodes 21 and 25 fora period of 10 seconds, temperatures within the sidewall of the parison11 are predicted which are shown in Table II. By experimentalmodification of these dimensions we obtained an electrode 21 of thedimensions of FIG. 1 by means of which parisons were properly heated forinflation into bottles more uniformly oriented in the sidewall thereofthan would have been obtained from the blow-molding of similar parisonsheated conventionally. The dimensions of this electrode can be seen fromFIG. 1 to differ by not more than five per cent from the predictedvalues. The parisons were heated in the arrangement of FIG. 1 whereinthe source of radio frequency energy was the model L14E generatordesigned to operate at 30 megacycles per second previously described.The generator was set to provide about 56 percent of maximum power forabout 10 seconds.'Since the voltage across the electrodes at a givenpower setting is dependent upon the state of tuning of the generator, aprocedure for establishing the voltage across the electrodes is given asfollows:

TABLE II Distance Estimated From Dimensions Predicted Temperatures (2)Open End of Central Within Parison Sidewall Electrode lnner Middle OuterSurface Surface l.000 0.730 I83 I85 I83 L375 0.705 186 188 I86 2.5050.655 180 186 I81 2.945 0.555 211 I96 I 3.564 0.480 219 I91 167 4.4370.400 29l I95 157 4.916 0.370 315 I91 150 All Dimensions in inches. Alltemperatures in Fahrenheit degrees.

A Peak Radio Frequency Kilovoltmeter as supplied by Thermatron Div.,Solidyne Corp., Bayshore, N.Y., is connected to the electrode to whichhigh voltage is applied, e.g., electrode 25, and is grounded. Theconnection to the electrode 25 should be straight, solid copper wire.Voltages should be measured at several power settings and for severallengths of connector. The voltage at zero-length of connector may thenbe determined by graphical extrapolation for any power setting, and thedesired power settings thereby determined by interpolation. Thezero-lead length voltage corresponding to the setting of 56 percent ofmax. power at the state of tuning employed in the heating of theparisons described above was about 4300 volts. That the voltage measuredby this means need not correspond to the actual voltage applied duringthe heating of parisons will be understood to result from an interactionof the measuring means with the state of tune of the generator.

It should be noted that although the drawings indicate air gaps betweenthe parison 11 and the electrodes 21 and 25, that dielectrics other thanair could also be employed to replace or to partly replace these airgaps. Thereby it is possible to modify further the intensity of thefield generated between the electrodes. It is generally not desirable,for most parison shapes, to utilize the electrodes 21 and 25 in completecontact with the parison 11 on its entire inner and outer surfaces.

The instant invention is applicable for any bottle sidewall thickness,but is especially useful for thicknesses between 5 and 60 mils, andparticularly between 15 and 35 mils. The invention is also applicablefor any parison, especially those whose sidewall thickness in an axialzone being heat treated is between and 250 mils. Temperature gradientsbetween 10 and 250 F. have been found useful, and particularly usefulare gradients between 40 and F.

The molecular orientation temperature range of an essentiallynon-crystalline thermoplastic polymer useful in the practice of thepresent invention is that temperature range above the glass transistiontemperature in which the polymer is rubbery or leathery. The highestdegree of molecular orientation is obtained by stretching the polymerwhen it is in the leathery state, viz. where its behavior is retardedhighly elastic, which is evidenced when the polymer, when subjected to astress, undergoes a small, instantaneous strain followed by a muchlarger strain over a relatively long period of time. The orientationtemperature range of the polymer described in the examples is about F toabout 275 F.

It is thought that the invention and many of its attendant advantageswill be understood from the foregoing description and it will beapparent that various changes may be made in the steps of the process(method) described and their order of accomplishment without departingfrom the spirit and scope of the invention or sacrificing all of itsmaterial advantages, the process (method) hereinbefore described beingmerely a preferred embodiment thereof.

What is claimed is:

l. A method of heat treating an axial zone of a polar, dielectricallylossy, thermoplastic, polymeric parison and producing a temperaturegradient across the thickness of the sidewall in said zone prior to blowmolding the parison into a bottle, comprising:

locating said parison between concentrically arranged electrodes; and

dielectrically heating the parison with a high frequency energy sourceto thereby produce said tem perature gradient in said zone wherein thetemperature is higher on the inner surface of the parison sidewall thanon the outer surface of the parison sidewall.

2. The method of claim 1 wherein the temperature gradient in Fahrenheitdegrees in said axial zone is in the range from about to about to aboutwhere SR(i) is the ratio of the inner diameter of the bottle in saidzone to the inner diameter of that portion of the parison from whichsaid zone of the bottle was formed, and where SR(o) is the ratio of theouter diameter of the bottle in said zone to the corresponding outerdiameter of the parison.

4. The method of claim 3 wherein the temperature gradient in Fahrenheitdegrees in said axial zone is determined according to the formula:

5. The method of claim 3 wherein the concentrically arranged electrodesare circular in cross section.

6. The method of claim 3 wherein the thickness of the sidewall in saidaxial zone is between 100 and 250 mils.

7. The method of claim 3 wherein the temperature gradient is between and250 Fahrenheit.

8. The method of claim 3 wherein there is an air gap between the parisonand the elctrodes.

9. The method of claim 8 wherein the air gap is filled or partiallyfilled with one or more dielectric materials other than air.

10. The method of claim 1 wherein the polymer is derived from (A) about60 to 90 parts by weight of an alpha,beta-olefinically unsaturatedmononitrile having the structure CH C(R,)--CN where R is selected fromthe group consisting of hydrogen, halogen, and lower alkyl groups, (B)about 40 to 10 parts by weight of an ester of an olefinicallyunsaturated carboxylic acid having the structure CH C(R,)C(O)OR where R,is as defined above and --R is an alkyl group having from 1 to 6 carbonatoms, (A) and (B) together comprising 100 parts by weight, polymerizedin the presence of (C) about 1 to parts by weight of a nitrile rubbercontaining about 60 to 80 percent by weight of moieties derived from aconjugated diene monomer and about 40 to 20 percent by weight ofmoieties derived from a mononitrile having said CH C(R )CN structure.

11. The method of claim 10 wherein the mononitrile is acrylonitrile'ormethacrylonitrile, the ester is selected from the group consisting ofmethyl acrylate, ethyl acrylate and methyl methacrylate, and theconjugated diene monomer is butadiene or isoprene.

12. The method of claim 11, wherein the polymer is derived from 73 to 77parts by weight acrylonitrile and 27 to 23 parts by weight of methylacrylate, polymerized in the presence of 8 to 10 additional parts byweight of a nitrile rubber containing about percent by weight butadienemoieties and about 30 percent by weight acrylonitrile moieties.

13. The method of claim 1 wherein the temperature gradient is betweenabout 40 and F.

14. The method of claim 1 wherein the parison is injection molded.

15. The method of claim 1 wherein the frequency is radio frequency.

16. The method of claim 15 wherein the frequency is between 10 and 10cycles per second.

17. The method of claim 1 wherein the polymer has a dielectric losstangent greater than 0.02 at room temperature measured at 30 megacyclesper second.

18. The method of claim 17 wherein the electrodes are half-hard brass.

1. A method of heat treating an axial zone of a polar, dielectricallylossy, thermoplastic, polymeric parison and producing a temperaturegradient across the thickness of the sidewall in said zone prior to blowmolding the parison into a bottle, comprising: locating said parisonbetween concentrically arranged electrodes; and dielectrically heatingthe parison with a high frequency energy source to thereby produce saidtemperature gradient in said zone wherein the temperature is higher onthe inner surface of the parison sidewall than on the outer surface ofthe parison sidewall.
 2. The method of claim 1 wherein the temperaturegradient in Fahrenheit degrees in said axial zone is in the range fromabout 25((SR(i)/SR(o)) - 1) to about 150((SR(i)/SR(o)) - 1) where SR(i)is the ratio of the inner diameter of the bottle in said zone to theinner diameter of that portion of the parison from which said zone ofthe bottle was formed, and where SR(o) is the ratio of the outerdiameter of the bottle in said zone to the corresponding outer diameterof the parison.
 3. The method of claim 1 wherein the temperaturegradient in Fahrenheit degrees in said axial zone is in the range fromabout 75((SR(i)/SR(o)) - 1) to about 125((SR(i)/SR(o)) - 1) where SR(i)is the ratio of the inner diameter of the bottle in said zone to theinner diameter of that portion of the parison from which said zone ofthe bottle was formed, and where SR(o) is the ratio of the outerdiameter of the bottle in said zone to the corresponding outer diameterof the parison.
 4. The method of claim 3 wherein the temperaturegradient in Fahrenheit degrees in said axial zone is determinedaccording to the formula: 100((SR(i)/SR(o)) - 1).
 5. The method of claim3 wherein the concentrically arranged electrodes are circular in crosssection.
 6. The method of claim 3 wherein the thickness of the sidewallin said axial zone is between 100 and 250 mils.
 7. The method of claim 3wherein the temperature gradient is between 10* and 250* Fahrenheit. 8.The method of claim 3 wherein there is an air gap between the parisonand the elctrodes.
 9. The method of claim 8 wherein the air gap isfilled or partially filled with one or more dielectric materials otherthan air.
 10. The method of claim 1 wherein the polymer is derived from(A) about 60 to 90 parts by weight of an alpha,beta-olefinicallyunsaturated mononitrile having the structure CH2 = C(-R1)-CN where R1 isselected from the group consisting of hydrogen, halogen, and lower alkylgroups, (B) about 40 to 10 parts by weight of an ester of anolefinically unsaturated carboxylic acid having the structure CH2 =C(-R1)-C(O)O-R2 where -R1 is as defined above and -R2 is an alkyl grouphaving from 1 to 6 carbon atoms, (A) and (B) together comprising 100parts by weight, polymerized in the presence of (C) about 1 to 20 partsby weight of a nitrile rubber containing about 60 to 80 percent byweight of moieties derived from a conjugated diene monomer and about 40to 20 percent by weight of moieties derived from a mononitrile havingsaid CH2 = C(-R1)-CN structure.
 11. The method of claim 10 wherein themononitrile is acrylonitrile or methacrylonitrile, the ester is selectedfrom the group consisting of methyl acrylate, ethyl acrylate and methylmethacrylate, and the conjugated diene monomer is butadiene or isoprene.12. The method of claim 11, wherein the polymer is derived from 73 to 77parts by weight acrylonitrile and 27 to 23 parts by weight of methylacrylate, polymerized in the presence of 8 to 10 additional parts byweight of a nitrile rubber containing about 70 percent by weightbutadiene moieties and about 30 percent by weight acrylonitrilemoieties.
 13. The method of claim 1 wherein the temperature gradient isbetween about 40* and 150* F.
 14. The method of claim 1 wherein theparison is injection molded.
 15. The method of claim 1 wherein thefrequency is radio frequency.
 16. The method of claim 15 wherein thefrequency is between 107 and 108 cycles per second.
 17. The method ofclaim 1 wherein the polymer has a dielectric loss tangent greater than0.02 at room temperature measured at 30 megacycles per second.
 18. Themethod of claim 17 wherein the electrodes are half-hard brass.